If your vegetable patch is measured in metres rather than acres, every square foot has to pull its weight. Companion planting is one of the few old allotment tricks that costs nothing, suits any layout, and genuinely earns its keep in a tight plot. The principle is straightforward: grow plants together that help one another, whether by masking each other's scent from pests, smothering weeds, or simply occupying different layers of the same bed.

Start with the classic pairings

Some combinations have been handed down for generations because they work. Two are worth building your whole plan around.

Lettuce with radishes. Radishes germinate in three or four days and are ready to pull in three to four weeks, which makes them the perfect marker crop. Sow a pinch of 'French Breakfast' or 'Cherry Belle' every 10cm along the row, then thin your lettuce seedlings around them. The radishes break the soil crust for the slower lettuce, mark the row so you can hoe without decapitating seedlings, and are lifted just as the lettuces begin to need the room. The scent of radish foliage also seems to muddle flea beetle, which otherwise riddles salad leaves with tiny shot holes in dry spring weather.

Carrots with onions. Carrot root fly and onion fly both hunt by smell, and each is confused by the other's foliage. Sow a row of carrot — 'Flyaway' and 'Resistafly' have some tolerance — with a row of onion sets such as 'Sturon' or a line of spring onions either side. Keep the rows about 15cm apart so the onions still get full sun. The one rule that matters more than any companion: thin carrots in the evening, on a still day, and take the thinnings straight off the plot. Crushed carrot foliage is what draws the fly in.

Let strong scents stand guard

Aromatic herbs do quiet work along the edges of a bed. Rosemary, sage, thyme and winter savory are all worth a place at the front of a sunny plot, and their oils seem to mask the scent of nearby brassicas from cabbage white butterflies. Keep mint in a pot sunk into the soil, or it will run everywhere.

• Nasturtium — a sacrificial trap crop for blackfly and cabbage white caterpillars. Grow it a metre away from your beans and let the aphids have it.
• French marigolds — the roots release compounds that suppress root-knot eelworm. Plant them around greenhouse tomatoes and between rows of potatoes.
• Borage — a bee magnet, and traditionally grown beside strawberries to improve flavour and fruit set.
• Phacelia — sown as a green manure or a strip along the path, it pulls in hoverflies whose larvae devour aphids.

Use the vertical layers, not just the ground

In a plot the size of a dining table, the cleverest thing you can do is stack your crops. The old three sisters arrangement — sweetcorn, climbing beans and squash — adapts beautifully to a small bed if you scale it down. Plant a block of four sweetcorn plants 45cm apart, not a single row, so they pollinate properly. Once they are knee-high, put a wigwam of climbing French beans such as 'Cobra' alongside, and let one courgette or a squash ramble across the ground beneath. The beans fix nitrogen, the corn gives the beans something to climb and shelter from wind, and the squash leaves shade the soil, keeping moisture in and weeds down.

Shade is an asset in July, too. Lettuce and spinach bolt in heat, so sow a row between tall peas or under the sweetcorn canopy. You will get tender leaves for weeks longer, and the space underneath would otherwise be wasted.

Flowers that pay their way

Leave a 30cm strip of annual flowers down one side of the plot and you will notice the difference by midsummer. Sweet alyssum is small enough to tuck beneath brassicas, and its tiny white flowers bring in hoverflies and parasitic wasps that keep aphids and caterpillars in check. Calendula flowers for months, self-seeds gently, and its petals are edible. Chives and garlic chives in flower are superb for bees and double as a cut-and-come-again crop. None of these need a separate bed — a line along the edge is plenty, and it saves you netting the whole plot against pests.

Keep it simple, and rotate

Companion planting is a help, not a substitute for the basics. Feed the soil with compost each spring, water deeply rather than often, and net against pigeons if they find you. Rotate your groups so alliums do not follow alliums and brassicas do not follow brassicas — a three-year cycle is enough in a small plot.

A few plants are best kept apart. Fennel releases chemicals that suppress its neighbours, so grow it in a pot. Dill and coriander can inhibit carrots if sown too close, and tomatoes and potatoes share the same blight risk, so give them separate corners. Otherwise, experiment. Keep a notebook of what grew well together, and within two seasons you will have a planting plan tailored to your own soil, your own pests, and your own small, productive patch.